coolant crossover leak

Just registered & have a question on a 67 deville conv w/ a 429 engine.
I own an auto repair shop and this car is a regular , it came in w/o oil pressure & we removed timing cover [ w/ an extreme amount of potholes in the removal , bolts corroded into cover ] & replaced oil pump gears , timing chain & gears & water pump , after all the obsticals we ended up w/ a coolant leak at water crossover were it plugs into timing cover. I sand blasted the nipple & cleaned it up real well , it apears as though the o-ring may be slighty undersized.

Does anyone have a solution w/o just siliconing it up and hoping it doesn't leak ??????????